1. Field of the Invention
The present invention relates to a process for preparing semicarbazide hydrochloride.
2. Description of the Prior Art
Semicarbazide hydrochloride is an important hydrazine-based chemical intermediate used in the production of pharmaceuticals, photographic chemicals and other products. This compound has been prepared by a variety of synthesis routes. One preparation method involves reacting carbon monoxide with hydrazine at superatmospheric pressures and at temperatures in the range from 0.degree. C. to 200.degree. C. and in the presence of a catalytic amount of a metal carbonyl to obtain semicarbazide which was acidified to yield the desired salt. See U.S. Pat. Reissue No. 24,526, which reissued to H. J. Sampson, Jr. on Aug. 26, 1958.
Another method involves reacting nitrourea with hydrogen in the presence of a hydrogenation catalyst, hydrochloric acid and an inert solubilizing agent to produce semicarbazide hydrochloride. See U.S. Pat. No. 2,749,217, which issued to A. J. Deutschman, Jr. on June 5, 1956.
A preferred commercial route involves the reaction of hydrazine hydrate (64% by weight hydrazine) with urea. At the end of the reaction, the water and unreacted hydrazine from the hydrazine solution is stripped. The reaction mixture is then digested in methanol followed by filtering off methanol-insoluble by-products (e.g. hydrazodicarbonamide). The remaining filtrate is acidified with aqueous hydrochloric acid to precipitate semicarbazide hydrochloride which is then recovered.
This latter route, while being advantageous in being able to use atmospheric pressure reactions and inexpensive reactants, does have the disadvantage of having relatively low yields (i.e. about 75% to 80% based on the urea used). Accordingly, there is a need in the art to be able to improve the yields of this process without substantially increasing its costs. The present invention is believed to be a solution to that need.